Fats and oil and fatty acid esters derived therefrom generally contain at least several to several tens of ppm of sulfur. The terminology, "fats and oils", as used herein means triglycerides and the terminology, "fatty acid esters", as used herein means esters, except triglycerides, of fatty acids and lower or higher alcohols. The raw materials will hereinafter sometimes be referred to as "starting oils" or "starting esters", inclusively. When the sulfur-containing starting oils are catalytically reduced with hydrogen in the presence of a catalyst for ester reduction to produce corresponding alcohols, a trace amount of sulfur compounds present therein acts as a catalyst poison to cause a marked reduction of duration of catalyst life.
The inventors made a study of purification methods for the starting oils to reduce the sulfur content thereof and, as a result, brought out the following points.
1) Problems of Purification by Distillation
When methyl esters of fatty acids which are derived from natural fats and oils in a conventional manner are subjected to distillation to a yield of 90% or 98%, the sulfur content can be reduced to 10% or 20%, respectively, of the initial content. However, where fatty acid methyl esters which are usually available or prepared are distilled for meeting the purpose of sulfur content reduction, there is an unavoidable loss of at least 5%, and the alkyl distribution of the starting material varies largely.
In the case of fats and oils or esters of fatty acids and higher alcohols, because of the high boiling point thereof, it is difficult to remove the sulfur compounds from such starting materials by distillation.
2) Problems of Purification with Catalyst for Desulfurization
In the field of petroleum refining, molybdenum or tungsten catalysts are used for removing sulfur compounds from light oil and heavy oil (refer to Shokubai Process Kaqaku, Tokyo Kagaku Dojin Shuppan).
The catalysts require temperatures of 300.degree. C. or higher for manifestation of the desulfurization activity. If fats and oils or fatty acid esters are hydrogenated in such high temperatures, hydrogenolysis of the ester group is attended by an increase in acid value and a marked increase of decomposition products of the starting material. In addition, a catalyst component is dissolved by the produced fatty acids which adversely affects selectivity of the catalyst in the ester reduction reaction.